I wouldnt worry about it.
I wouldnt think it would give it a watery flavor.. but a lighter body.
No matter what it will still be beer
I got an extra 1/2 gallon in my first batch during the bottling phase.. (Long story, one of these days I will read directions better). Made my stout watery at first. But once the yeast carbed up the beer, and it aged a bit the watery flavor went away.
Since yours was added before fermentation, I would think you will be fine.